Rewrite evbuffer_expand and its users

Rewrite evbuffer_expand and its users

The previous evbuffer_expand was not only incorrect; it was
inefficient too.  On all questions of time vs memory tradeoffs, it
chose to burn time in order to avoid wasting memory.  The new code
tries to be a little more balanced: it only resizes an existing chain
when doing so doesn't require too much copying, and when failing to do
so would waste a lot of the chain's space.

This patch also rewrites evbuffer_chain_insert to work properly with
last_with_datap, and adds a few convenience functions to buffer.c.